Silicon is unique in photovoltaic materials in that it is an elemental semiconductor with no stoichiometry and interdiffusion problems. Crystalline silicon solar cells have high efficiency and excellent stability. Currently, about 72% of the world's photovoltaic module shipments consist of single or polycrystalline silicon solar cells. Crystalline silicon can tolerate high-temperature device processing steps, and, indeed, requires many high-temperature steps for high-efficiency solar cell manufacturing. These high-temperature steps have been the dominant factor for the high cost of silicon solar cell manufacturing.
The present invention uses concentrated sunlight instead of electricity, as the energy source for the high-temperature steps needed to manufacture silicon solar cells. When concentrated sunlight strikes a surface, such as a silicon wafer, the surface is rapidly heated to accomplish a variety of thermal processing steps, and can be cooled in an easily controlled manner.
Sunlight is concentrated using a high flux solar furnace. Generally, a high flux solar furnace is comprised of a heliostat to track the movement of the sun, a concentrator, and a target point. The concentrator can be one or more mirrors, or, one or more refractive lenses, aligned so as to direct the incident sunlight at a predetermined focal point. This focal point is selected to maximize the concentration of incident light, or achieve a desired flux profile. The target point coincides with this focal point, and a platform or chamber is placed at this focal point to support the material to be processed.
Most electricity is currently generated by converting conventional sources, such as fossil fuels and nuclear power, to heat, and then from heat to electricity. This electricity is then converted back into heat for high-temperature processing of solar cells. This inefficient method results in considerable transmission and generation losses, and excessive pollution. The concentrated sunlight of the present invention is provided by a solar furnace, which produces high temperatures directly from the radiant energy of concentrated sunlight. In addition, the heating rate is extremely high, with the concentrated sunlight often being able to heat thin layers of the surface being treated to the desired processing temperature in fractions of a second, while leaving the bulk unaffected. Also, because a solar furnace can deliver photons from the entire solar spectrum, preferred portions of the spectrum can be selected by using the proper filters. Also, a solar furnace relies on an abundant and clean supply of energy.
The prior art teaches silicon solar cell processing by using heating lamps, such as tungsten-halogen quartz lamps that are rich in infrared, and lasers, such as excimer lasers that are rich in ultraviolet. Flash lamp heating has already been used in commercially available products for firing screen printed contacts for silicon (Si) solar cells and for soldering metal ribbons for interconnecting cells to form modules.
An important difference between using a high flux solar furnace and artificial light sources is that the solar furnace offers a slow and easily controllable cool-down rate. Rapid thermal processing (RTP), as disclosed by Hartiti, B. et al (Progress in Photovoltaics, Research and Applications, Vol. 2, pp. 129-142, 1994), states that "Rapid thermal processing has only little success for devices like power diodes, nuclear detectors, and solar cells because the problem for these kinds of devices is the preservation of high bulk lifetimes. This last point has been the major source of drawbacks of early solar cells (fabricated) using rapid thermal processing . . . The origin of this degradation is related to transition metals or metallic complexes involving carbon, oxygen and the dopant which are not deactivated during the quenching step inherently associated to the fast cooling (of the rapid thermal processing process)." While fast cooling is inherent to rapid thermal processing using artificial sources, it is not inherent to a high-flux solar furnace. Slow, controllable cooling, as is achieved using a high flux solar furnace, is believed to contribute to the improved performance of high flux solar furnace-processed cells over conventional cells. In addition, it is very easy to control the cooling rate in a high flux solar furnace. Rapid thermal processing for solar cells has been studied for at least over ten years, and has not been adopted in a large scale by the solar cell industry.
Another advantage of using concentrated sunlight is the very broad (wide) wavelength spectrum of the high flux solar furnace compared to most artificial sources. Many of the artificial light sources used for solar cell processing are "single-wavelength" lasers. A broad wavelength spectrum provides more uniform heating through the thickness of the solar cell. This uniform heating is important for solar cells because, unlike integrated circuits which are planar devices, solar cells need to have electric charge transported through the entire thickness of the cell. A planar device like an integrated circuit uses only the top surface region of the device.
Artificial light sources, such as arc lamps, can produce broad spectra, but usually have very short electrode life and are not spatially uniform enough for large area solar cell processing. Solar cells are "large-area" devices. i.e. typically 10 cm.times.10 cm in area, or larger. Some commercial solar cells are as large as 1.2 m.times.1.2 m (4 ft.times.4 ft). It is not easy to make high-intensity artificial light sources having even distribution over such large areas. A major advantage of using a solar furnace over arc lamps is that the long focal length of the solar furnace permits substantial flexibility in the geometry of the set up which is not possible with the characteristically short focal length of an arc lamp.
Another advantage of the process of the present invention is that it is a cold-wall process; i.e., only the sample under treatment is heated, not the reactor walls. This greatly reduces the possibility of impurity contamination from the chamber walls. Impurity contamination from the chamber walls has been a major factor in reducing the carrier lifetime of silicon in conventional high-temperature processes. This in turn reduces the performance of the solar cells manufactured. Periodic, sometimes daily, cleaning of the high-temperature furnace using hydrofluoric acid-containing solutions is required. This increases the cost of manufacturing solar cells and also generates large quantifies of toxic waste. In conventional hot-wall furnace processing, the need to heat up the entire furnace is a tremendous waste of energy, and also limits the throughput because it is a time consuming process to heat up and cool down high-temperature furnaces. In contrast, high flux solar furnace processing rapidly and efficiently heats only the sample directly with concentrated sunlight. And, because the reactor walls remain cool, the chances of sample contamination from impurities emanating from the reactor walls are greatly reduced. This method of manufacturing high-efficiency solar cells also reduces the cost of manufacturing because less frequent cleaning of the reactor is needed.
In summary, the high-intensity, large-area light flux and the high degree of control over the flux profile that can be achieved using a high flux solar furnace cannot be readily achieved using any single or combination of artificial light sources currently available. Because of its simplicity, the high flux solar furnace can easily be modified for a variety of manufacturing applications.
Another silicon solar cell process which is recently gaining importance and which can be effectively accomplished using the high flux solar furnace is the crystallization of amorphous silicon thin films into multicrystalline silicon. Multicrystalline silicon is defined herein as being non-amorphous silicon.
Multicrystalline silicon thin films (mc-Si) are usually deposited on a substrate by a chemical vapor deposition (CVD), a physical vapor deposition (PVD), or a liquid-phase epitaxy (LPE) technique. Si films deposited by physical vapor deposition or chemical vapor deposition on a low-cost, nonepitaxial substrate usually have an average grain size of less than 1 micron. Microcrystalline (.mu.c-Si) silicon is defined herein as having an average grain size on the order of 1 micron, or less. Microcrystalline silicon films deposited using plasma-enhanced chemical vapor deposition with strong hydrogen dilution of the Si-containing feedgas and high radio-frequency (RF) power densities are widely used as heavily doped window contact layers and tunnel-junction contact layers in hydrogen containing amorphous silicon (a-Si:H) solar cells. (Luft, W. and Tsuo, Y. S. 1993, Ch. 15, Hydrogenated Amorphous Silicon Alloy Deposition Processes, Marcel Dekker, Inc., New York, 1993 and Tsuo et al., "Solar Cell Structures Combining Amorphous, Microcrystalline, and Single-Crystalline Silicon," Proc. 23rd IEEE Photovoltaic Specialists Conf., pp. 281-186, 1993). However, larger Si grain sizes are needed for use as active semiconductor layers in such applications as thin film transistors for controlling active-matrix liquid-crystal picture elements (pixels) of large-area flat-panel displays. (See, for example, Powell, M. J., IEEE Transactions on Electron Devices Vol. 36, p. 2753, 1989, and Kanicki. J., Editor, "Amorphous & Microcrystalline Semiconductor Devices, Vol. II: Materials and Device Physics", Artech House, Inc., Boston, 1992) and photovoltaic solar cells (Meier, J., Fluckiger, R., Keppner, H. and Shah, A. "Complete microcrystalline p-i-n solar cell-crystalline or amorphous cell behavior?", Appl. Phys. Lett., Vol.65, pp. 860-862, 1994.) A post-deposition annealing of physical vapor deposition or chemical vapor deposition deposited amorphous silicon or multicrystalline silicon is usually needed to obtain large crystalline grain sizes. Plasma-enhanced chemical vapor deposition is usually used to deposit the initial amorphous silicon film because of its low temperature deposition and high purity. Relatively high deposition rates can be used because the initial electronic property of the amorphous silicon is not important, since the film will subsequently be crystallized.
Liquid-phase epitaxy silicon deposited on an epitaxial substrate usually has the high deposition rate and large grain size required for semiconductor device applications. However, the need for a substrate with large grain size and a lattice constant that closely matches that of the deposited silicon and the relatively high deposition temperature are disadvantages of liquid-phase epitaxy silicon growth. For liquid-phase epitaxy silicon films deposited on a low-cost, nonepitaxial substrate, a post-deposition annealing technique to enlarge the grain size is also needed. It is also important to note that many low-cost substrates, such as Corning 7059 glass, cannot stand processing temperatures above 600.degree. C. Thus, a low-temperature, solid-phase crystallization (SPC) method is often preferred.
For conventional furnace annealing of plasma-enhanced chemical vapor deposition deposited a-Si:H, grain size as large as 4 .mu.m and field effect mobility as high as 158 cm.sup.2 /V.multidot.s have been reported by Nakazawa, K. and Tanaka, K., J. Appl. Phys. Vol. 68, p. 1029, 1990. For comparison, the effective electron mobility in a-Si:H is only up to 1 cm.sup.2 /V.multidot.s; in single-crystal Si, it is up to 1400 cm.sup.2 /V.multidot.s. However, such annealing usually takes over ten hours or even days and cannot be used to selectively crystallize certain areas of an a-Si:H film. Sanyo Electric Company recently produced an 8.5%-efficient, 1-cm.sup.2 solar cell using a 10-.mu.m-thick, solid-phase crystallization multicrystalline silicon with the structure of indium tin oxide (ITO)/p-type a-Si:H/intrinsic a-Si:H/n-type .mu.c-Si/metal. The solid-phase crystallization poly-Si deposited on a textured substrate has a maximum mobility of 623 cm.sup.2 /V.multidot.s at a carrier concentration of 3.10.times.10.sup.15 cm.sup.-3. The solid-phase crystallization was done in a conventional furnace at 600.degree. C. for 10 hours. (Matsuyama, T., Baba, T., Takahama, T., Tsuda, S., and Nakano, S., "Polycrystalline Si thin-film solar cell prepared by solid phase crystallization method", Solar Energy Materials and Solar Cells, Vol. 34, pp.285-289, 1994). This 8.5% conversion efficiency is higher than the stabilized efficiency of most a-Si:H solar cells.
Melt recrystallization methods using a directed energy beam are interesting alternatives that have the added advantage of being able to selectively crystallize areas of the deposited film. Conventional rapid thermal processing with a continuous-wave laser requires high substrate temperature and is not very practical since high substrate temperature can cause substrate damage and semiconductor film contamination. Rapid thermal processing also has high energy requirements. (Troxell, J. R., Harrington, M. I., and Miller, R. A., "IEEE Electron Device" Lett. Vol. EDL-8, p. 576, 1987).
More recent investigations involving using a pulsed laser, such as an excimer laser have achieved considerable success. (Sera, K., Okumura, F., Uchida. H., Itoh, S., Kaneko, S., and Hotta, K., "IEEE Transactions on Electron Devices" Vol. 36. p. 2868, 1989: and Bachrach, R. Z., Winer, K., Boyce, J. B., Ready, S. E., Johnson, R. I, and Anderson. G. B., J. Electronic Materials Vol. 19, p. 241, 1990). For example, Sera et al. reported that the field effect mobility increased from 0.23 cm.sup.2 /V.multidot.s for a-Si:H to 102 cm.sup.2 /V.multidot.s for laser-crystallized .mu.c-Si. Recently, solar cells with 6.5% conversion efficiency have been made using 4.2-.mu.m-thick laser-melt-and-crystallization silicon with optical confinement by diffuse reflective substrate (Shimokawa. R., Ishii, K., Nishikawa, H., Takahashi, T., Hayashi, Y., Saito, I., Nagamine, F., and Igari, S., "Sub-5 .mu.m Thin Film c-Si Solar Cell and Optical Confinement by Diffuse Reflective Substrate," Solar Energy Materials and Solar Cells, Vol. 34, pp. 277-283, 1994). Instead of a laser beam, an ion beam can also be used for crystallization. High-energy (1.5 MeV) Xe.sup.+ ion irradiation has been shown to enhance the solid-phase crystallization of a-Si:H at low temperatures (500.degree.-580.degree. C.). (Im, J. S. and Atwater, H. A. Appl. Phys. Lett., Vol 57, p. 1766, 1990).
Zone-melting recrystallization (ZMR) by scanning a carbon strip heater is also a popular method of crystallizing silicon. Recently, a 14.2% photovoltaic solar cell has been made using a 60-.mu.m-thick active polycrystalline silicon layer with n-millimeter grain size. The polycrystalline silicon film was originally deposited on a SiO.sub.2 -coated single-crystalline silicon wafer by low-pressure chemical vapor deposition (Arimoto, S., Morikawa, H., Deguchi, M., Kawama, Y., Matsuno, Y., Ishihara, T., Kumabe, H., and Murotani, T., "High-efficient Operation of Large-area (100 cm.sup.2) Thin Film Polycrystalline Silicon Solar Cell Based on SOI Structure," Solar Energy Materials and Solar Cells, Vol.34, pp.257-262, 1994).